Superconducting coils are used to generate strong, homogeneous magnetic fields, said superconducting coils being operated in a sustained short-circuit current mode. For example, homogeneous magnetic fields with magnetic flux densities between 0.5 T and 20 T are required for nuclear magnetic resonance spectroscopy (NMR spectroscopy) and for magnetic resonance imaging. These magnets are typically charged via an external electric circuit and are then separated from the external power source since, in the resulting sustained short-circuit current mode, a nearly lossless current flow occurs within the superconducting coil. The resulting, strong magnetic field is particularly stable over time since it is not affected by the noise contributions of an external electric circuit.
During the use of known winding techniques, one or more superconducting wires are wound on support bodies, wherein different wire segments are maintained in contact with one another by wire connections with optimally low ohmic resistance or via superconducting connections. For conventional low-temperature superconductors, such as NbTi and Nb3Sn with transition temperatures below 23 K, technologies exist for the establishment of superconducting contacts to link wire segments and to connect the windings with a superconducting sustained current switch. The superconducting sustained current switch is thereby part of the electrical circuit of the coil and is placed in a resistive conductive state by heating in order to inject an external current. After deactivating the heating and cooling down to the operating temperature, this part of the coil is also superconducting again.
High-temperature superconductors, also called high-Ta superconductors (HTS) are superconducting materials with a transition temperature above 25 K, and above 77 K for some material classes, such as cuprate superconductors with which the operating temperature can be achieved by cooling with cryogenic materials other than liquid helium. HTS materials are particularly attractive for the production of magnetic coils for NMR spectroscopy and magnetic resonance imaging, since many materials have high upper critical magnetic fields of over 20 T. Due to the higher critical magnetic fields, the HTS materials are in principle better suited than low-temperature superconductors for the generation of high magnetic fields of over 10 T for example.
One problem with the production of HTS magnetic coils is the absence of suitable technologies to produce superconducting HTS connections, in particular for second generation HTS, so-called 2G-HTS. The 2G-HTS wires are typically present in the form of flat strip conductors. If resistive contacts are introduced between the superconducting strip conductors, the losses in the coil can no longer be ignored and the generated magnetic field noticeably drops over a time period of a few hours or days.
DE 10 2010 042 598 A1 discloses a superconducting MR magnet arrangement comprising a superconducting strip conductor that is provided in the longitudinal direction with a slit between the two ends so that the superconducting strip conductor forms a closed loop surrounding the slit. The slit superconducting strip conductor is wound to form a multi-section coil made up of at least two partial coils that are arranged rotated counter to one another so that they generate a predetermined magnetic field curve in a measurement volume.
DE 10 2011 082 652 B4 also discloses a magnetic coil arrangement with a slit superconducting strip conductor and additionally a method for the production of such a magnetic coil arrangement. With the disclosed production method, the two half strips of the strip conductor are initially each wound onto an intermediate coil. Then, turns of the first and second half strips are wound onto one common winding body in alternation from the intermediate coils.
These winding methods described in the prior art have the drawback that the winding of the partial coils from the conductor branches of the slit strip conductor is relatively complex. With the parallel conductor guidance suggested in application 102013207222.8, the observance of a constant winding tension for each conductor branch is difficult, since the two conductor branches may have slight deviations in their lengths and/or elasticity under tension. However, a uniform and constant winding tension is of great importance for the quality and the dimensional stability of the resulting coil winding. In particular, uniform winding tension is important in order to be able to produce a coil winding without inclusions and/or cavities. With the production method disclosed in DE 10 2011 082 652 B4 using two intermediate coils, the conductor branches are guided in parallel. In all variants of the production method disclosed, they are twisted with respect to each other and/or out of the coil winding, in other words, they are twisted about an axis in the direction of their longitudinal extension. This torsion results in stress on the superconducting strip which can result in damage to the materials and their electrical and/or mechanical properties.
A further drawback of the known winding methods consists in the fact that, when wound simultaneously, the two conductor branches have to be guided substantially next to one another and it is therefore difficult to insulate the individual conductor branches electrically from one another. Finally, early planning of the strip conductor length to be wound is necessary because the length of the slit strip conductor used for the winding predetermines the dimensions of the coil to be wound. The complete slit conductor length has to be wound even if, for example, deviations in the conductor thickness cause the desired number of turns or winding height to be fallen below or exceeded. The slicing of the strip conductor material and the processing of the strip conductor slit in this way can also easily result in quality problems and material damage.